1. Field of Endeavor
The present invention relates to the field of machine elements in conjunction with thermal machines, and more particularly to a sealing arrangement.
2. Brief Description of the Related Art
In thermal machines, such as gas turbines, or electromechanical machines, such as electric generators, the cooling of thermally loaded components is an essential parameter for the overall efficiency and the service life of the system. In most cases, the cooling medium is cooling air; however, steam from a steam generator can also be used for the same purpose. The subsequently disclosed subject matter is correspondingly applicable to all cooling media, regardless of the supply source, but is explained using the example of an air-cooled gas turbine.
In a gas turbine, air is compressed by a centrifugal or axial compressor from the ambient pressure to nominal pressure. After cooling of the hot parts of the combustion chamber, the main portion of the compressed air is used for combusting fuel in the combustion chamber. The rest of the compressed air is extracted at one or more points along, or at the end of, the compressor, and directed through cooling air passages to the hot parts of the gas turbine. In the turbine, the cooling medium is used for the internal or external cooling of the turbine components, such as stator blades and rotor blades. In addition, the cooling medium reduces the operating temperature of non-rotating and rotating components, such as the blade roots or rotor disks, which are exposed to large centrifugal forces.
Some of the air is also used for sealing purposes, particularly between rotating and stationary parts, by the air being purged through a gap into the hot gas passage of the turbine in order to prevent entry of hot gas and therefore local overheating. The seal plays an important role in the effective distribution and controlling of the cooling air. On account of mechanical and thermal stresses and the thermal expansion of the components during operation, however, the gaps which are to be sealed alter in their dimensions. In the stationary state of the machine, each rotating and non-rotating component is assembled with other parts, taking into consideration manufacturing and assembly tolerances and also the anticipated mechanical and thermal deformation of the components. The cold gaps which therefore result enable the unhindered thermal expansion and rotation-induced deformation of the components during operation by the gaps which result in the process, the so-called hot gaps, at no time adopting unacceptably small values. The unhindered expansions and deformations prevent mechanical defects in each component where they adjoin other components.
Under operating conditions, depending upon the thermal and mechanical load of the respective component, the resulting hot gaps can become smaller or larger than the cold gaps relative to the stationary state of the machine. An example of such changes in a gas turbine is shown in FIGS. 1-3. FIG. 1 shows in a detail a typical arrangement of rotor blades and stator blades in a gas turbine, including the seals—in this case exemplarily of a strip-like construction—which are arranged between the components. The gas turbine 10 of FIG. 1 includes a rotor 11 which is equipped with rotor blades 13 and is enclosed by a turbine casing or stator blade carrier 12. The rotor blades 13 have an (inner) platform 14 (14′), beneath which the blade merges into a shank 15, on the end of which is arranged a blade root 16 by which the rotor blade 13 is fastened in the rotor 11. Stator blades 18, which have a(n) (outer) platform 19, are attached on the turbine casing or stator blade carrier 12. The blade airfoils of the rotor blades and stator blades 13, 18 lie in the hot gas passage of the turbine.
For sealing in relation to the hot gas passage, provision is made for seals—in this case exemplarily constructed as sealing strips 20, 21 (FIG. 2)—which extend in the axial direction (z-axis in FIG. 2) and in the circumferential direction (u-axis in FIG. 2), but can also extend in the radial direction and be arranged between adjacent platforms 14 (14′) of blades or between blades and adjacent heat shields 13, 17 or ring segments, as well as between adjacent heat shields (when provided). According to FIG. 3a, the sealing strips 21 lie transversely to the gap 22 between the platforms 14 and 14′ of adjacent components in a corresponding recess 23.
As exemplarily indicated in FIG. 3b, the gap between the rotor blades of each turbine stage increases during operation. Under the influence of centrifugal forces, the rotor blades are stretched in the radial direction, which results in a larger gap c in the circumferential direction between the platforms of the blades.
This enlargement of the gap in the circumferential direction is partially compensated for by the thermal expansion of the blade platforms. Depending upon the blade mass and the operating temperature, the thermal deformations of the platforms are usually less than the blade deformations which are brought about as a result of rotation. Therefore, the gap c between the blade platforms of longer blades in the circumferential direction during operation can be either larger than the cold gap c0 in the stationary state of the machine (FIG. 3a), or it can be smaller, which applies typically to non-rotating components or light rotor blades, the deformations of which are principally determined by the thermal load. In general, during operation the gaps can vary considerably depending upon the mechanical and thermal load of the rotating and non-rotating components of the machine. Furthermore, the radius of the platforms 14, 14′, which in the stationary state can have the value R0 (FIG. 3a), can acquire another value R (FIG. 3b) during operation.
In order to passively control the gaps during operation of the machine, according to FIG. 2 provision is made for axially and circumferentially extending seals—in this case exemplarily constructed as strips 20 or 21—which prevent an uncontrolled leakage of the cooling medium into the hot gas passage of the turbine. The seals 20, 21 generally are formed of an alloy which is suitable for the operating conditions of the machine.
The seals which are arranged in rotating components are pressed against the platforms of the rotor blades as a result of centrifugal forces. Consequently, a mechanical contact between the upper side of the seal 21 and the outer flanks of the seal groove 23 is created inside the platforms or heat shields 14, 14′, which is schematically shown in FIG. 3b. On account of the thermal expansion and rotation-induced deformations, the effective hot gap c (FIG. 3b) becomes larger during operation, whereas the width b of the seal 21 remains practically unaltered on account of its small dimensions relative to the size of the blade, of the rotor and of the heat shields. The mechanical contact which is created can be limited to a narrow contact area on the seal which does not lead directly to the best possible sealing effect or can even lead to undesirable or increased leakage on account of local deformations of the platform.
In the prior art, considerations have already been made to purposefully control sealing conditions in thermal machines by the use of memory alloys.
An arrangement of a rotor and a stator of a turbine, in which rotor blades with a blade airfoil and (inner) platform and stator blades with a blade airfoil and (inner) platform alternate with each other, is known from printed publication US 2007/0243061. Between the platforms of the rotor blades and the platforms of the stator blades a seal is defined, wherein the platforms of the rotor blades and/or stator blades in the region of the seal partially are formed of a memory alloy in order to control the cooling air flow through the seal in dependence upon temperature. Such controlling through the platforms themselves is extremely costly in production and dimensioning, because the blades themselves have to be constructed from different material and correspondingly prefabricated.
U.S. Pat. No. 7,086,649 B2 discloses an annular seal for inserting between two parts which rotate relatively to each other. The seal includes a section which can bend for altering the sealing gap. Such a section can be formed of a bimetal or a memory alloy in order to alter the gap width in a temperature-controlled manner.
A device for controlling the clearance between the blade tip of a rotor blade and the opposite wall of an axial turbine, in which the radial position of the wall is controlled by a spiral spring, formed of a memory alloy, which shifts the wall, is known from printed publication JP58206807.
The seals which are described in the introduction are not the subject of known proposals.